The following describes how you can determine which kind of node or edge will be created by clicking and/or dragging in the Editor window.
Default Node and Default Edge
With the standard settings yEd will create square nodes with yellow/orange color, if you click onto the background in the editor window in yEd. Pressing the left mouse button over a node, dragging the mouse and realeasing the mouse button over another node creates an edge connecting these nodes with a thin black line and an arrowhead pointing to the target node.
The template for nodes created as described above is called the default node. Similarly, the template for edges created as described above is called the default edge.
You can also create nodes and edges by dragging and dropping symbols from the Palette. The default node and the default edge do not apply in this case.
Changing the Defaults
In order to get differently looking nodes when you click in the editor window just double click a node symbol in the Palette view or use the entry As Default from its context menu. In the Palette it is visually indicated that the symbol you just clicked is the new default. Afterwards new nodes created as described above will be copies of the default node. The same holds for edges. There is a section Edge Types in the Palette which offers some possibilities for the default edge.
If the Palette does not contain a symbol which you want to use as the default for a node, you can change the properties of an existing node in the editor window and then add it to the section User Defined of the Palette (by using the entry Add to Palette from its context menu). Once you added the node to the Palette you can use it as the default as described before. The same holds for edges.
There is a difference between normal nodes, open group nodes and closed group nodes. If for example you double click an open group node symbol in the Palette it will be used as the default for open groups, but it won't become the default for normal nodes.
Saving the Defaults
yEd automatically saves the current defaults when you leave it. yEd restores the defaults when you start it, so your choice of defaults does not get lost by leaving yEd.
Resetting the Defaults
If you want to return to the initial defaults for nodes and edges, you can do so by choosing the entry Reset Defaults from the context menu of an arbitrary symbol in the Palette.